The field of the invention is magnetic resonance imaging (MRI) systems and methods. More particularly, the invention relates to the quantitative assessment of regional myocardial contractile function by producing strain maps from acquired MRI data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a roster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear” or a “Cartesian” scan. The spin-warp scan technique is discussed in an article entitled “Spin-Warp MR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space as described, for example, in U.S. Pat. No. 6,954,067. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory. Such pulse sequences are described, for example, in “Fast Three Dimensional Sodium Imaging”, MRM, 37:706-715, 1997 by F. E. Boada, et al. and in “Rapid 3D PC-MRA Using Spiral Projection Imaging”, Proc. Intl. Soc. Magn. Reson. Med. 13 (2005) by K. V. Koladia et al and “Spiral Projection Imaging: a new fast 3D trajectory”, Proc. Intl. Soc. Mag. Reson. Med. 13 (2005) by J. G. Pipe and Koladia.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1 DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection.
The ability to accurately quantify regional myocardial contractile function in the clinical setting has profound consequences in the clinical management of patients whose myocardial pathological processes demonstrate, and whose clinical course is dependent upon, regional and transmural variances in myocardial contractile function. Most notably, this impacts the clinical management of patients with ischemic cardiomyopathy secondary to atherosclerotic coronary artery disease. The most common clinical manifestation of this disease process is intermittent or permanent impairment of myocardial contractile function in the regions of the left ventricle whose coronary arterial blood supply is directly compromised by atherosclerotic occlusive disease. It is well established that the degree of impairment, the quantity of myocardium that is impaired, and the reversibility of the impairment all have a direct influence on clinically observed morbidity and mortality. In addition to ischemic cardiomyopathy, the quantification of regional myocardial contractile function is also an important tool in the clinical assessment of disease processes once felt to be regionally homogeneous, such as cardiomyopathy secondary to valvular or idiopathic myopathic processes.
In general, a more accurate characterization of regional and transmural contractile function impacts the investigation of human pathophysiological processes that directly affect the myocardium, such as the cardiomyopathies that accompany valvular, coronary arterial, and primary myocardial diseases. For example, the uniform nature of the circumferential myocardial thinning and symmetrical short axis cross-sectional shape that characterizes the left ventricular geometry of patients with non-ischemic, non-valvular dilated cardiomyopathy has long suggested a homogeneous myocardial process of injury. It is now recognized, however, that the pathological influence of secondary remodeling on both adjacent and distant non-injured myocardium can render uniform and homogeneous global ventricular geometrical changes from a heterogeneous injury process. In fact, previous investigations have suggested heterogeneous contractile changes in patients having non-ischemic, non-valvular dilated cardiomyopathy.
Catheter-based or cardiac surgical coronary revascularization procedures are the most commonly applied therapeutic interventions in patients with ischemic cardiomyopathy. Although many clinical factors are important in directing these high-risk interventions, the presence of viable myocardium in the distribution of target atherosclerotic vessels remains the primary factor in determining if intervention is warranted. Revascularizing viable myocardium improves outcomes; however, high-risk revascularization procedures directed at nonviable myocardium do not improve outcomes over medical therapy alone. Improving the accuracy of the detection and full characterization of the viable myocardium is, therefore, a critical factor in improving the outcomes in this high-risk patient population.
Currently utilized viability testing methodologies, such as thallium single-photon emission computed tomography (SPECT) imaging, positron emission tomography (PET) imaging, delayed-enhancement magnetic resonance imaging (MRI), and dobutamine echocardiography are effective in directing clinical decision-making. However, these imaging modalities are limited in the accuracy of their regional and transmural characterization of myocardial viability as a result of the qualitative nature of the images they produce. This qualitative image output predisposes to variability in the interpretation made by clinicians of regional involvement in irreversible ischemic injury. For example, multiple clinicians may variably characterize an area of nonviable myocardium that resulted from left anterior descending coronary artery occlusion and which overlaps anterior, anterolateral, and anteroseptal regions, as well as extending a variable distance from base to apex in each of these left ventricular (LV) regions.
In a similar fashion, quantification of the degree of transmural injury is also limited with current modalities. Myocardial ischemia resulting from coronary artery occlusive disease is well known to affect the myocardium in a non-uniform transmural distribution, with the subendocardium being more susceptible to ischemia than the epicardial myocardium. The ability to accurately differentiate isolated subendocardial infarction from transmural injury has important clinical implications in regard to prognosis, as well as the appropriate application of therapeutic intervention methods. Thallium SPECT imaging, PET imaging, delayed-enhancement MRI, and dobutamine echocardiography only offer a qualitative characterization of transmural inhomogeneity. Once again, a lack of quantification of transmural viability predisposes to inconsistency in clinician interpretation.
Magnetic resonance imaging has excellent spatial and temporal resolution and has the unique ability to track the systolic deformation of gridlines laid down by the radiofrequency tagging of myocardial tissue. For example, a pulse sequence such as the one described in U.S. Pat. Nos. 5,054,489; 5,217,016; 5,275,163; 6,171,241 and 6,721,589 can be utilized to produce a grid of dark lines on an MR image acquired at a reference cardiac phase and the deformation of these grid lines can be monitored in MR images acquired at other cardiac phases to determine the deformation of the patient's heart wall during a heart beat. This information, combined with excellent tissue boundary resolution without the need for intravenous contrast agents and the absence of radiation exposure, make MRI highly attractive as a clinically applicable imaging modality to assess regional differences in myocardial contractile function.
Many different MRI methods have been proposed to characterize left ventricle contractile function. These include methods disclosed in U.S. Pat. Nos. 6,757,423 and 7,030,874, as well as pending U.S. Patent Applications US2002/0176637 and US2008/0077032. These methods purport to calculate strain, torsion and other mechanical properties of myocardial function, but none disclose the calculation of a multiparameter strain index that correlates with abnormal contractile function.